1. Field of the Invention:
This invention relates to a bicycle speed change lever assembly, and more particularly to improvements in the lever assembly for operating a deraileur to shift a drive chain from one to another of diametrically different sprockets.
2. Prior Art:
As is well known, bicycles are equipped in many cases with a speed change mechanism to enable cycling suited to a particular road condition or to a cyclist's desire. One typical speed change mechanism includes for example a rear deraileur of the type that comprises a shiftable cage rotatably supporting a pair of guide pulleys in engagement with a drive chain, and a control mechanism such as a parallelogrammic linkage mechanism for displacing the cage laterally of the multiple freewheel (rear gear) to shift the drive chain from one sprocket to another of the freewheel. Another typical speed change mechanism includes a front deraileur which comprises a pair of parallel guide plates arranged on both sides of the drive chain, and a control mechanism such as a parallelogrammic linkage mechanism, similar to that of the rear deraileur, for displacing the guide plates laterally of a multiple chainwheel (front gear) to shift the drive chain from one sprocket to another of the chainwheel.
The movement of the shiftable cage or the guide plates is controlled by a control lever operatively connected to the parallelogrammic linkage mechanism via a control cable which is always subjected to a tension by a return spring incorporated into the linkage mechanism. The control lever has a boss portion rotatably supported on a support shaft fixed to a suitable part of the bicycle frame, so that the lever is pivotally operable. An end part of the control cable extends along a cable winding groove formed on the outer periphery of the lever boss portion which is imparted a rotational resistance or friction enough to overcome the tension of the cable. Thus, the lever may be frictionally held at any pivotal position against the tension of the cable.
When the control lever is manually pivoted, the control cable is wound up along the winding groove of the lever boss portion or paid out therefrom to vary the spanning length of the cable. As a result, the parallelogrammic linkage mechanism is correspondingly deformed to bring the shiftable cage or the guide plates to a lateral position determined by the pivotal position of the control cable.
Apparently, the friction type control lever is pivoted steplessly. It is thus quite difficult for an unskilled cyclist to properly operate the lever so as to bring the shiftable cage or the guide plates to an optimum position relative to each sprocket of the multiple freewheel or the multiple chainwheel. In fact, the cage or the guide plates may be erroneously brought to a position intermediate two adjacent sprockets, failing to conduct intended shifting of the drive chain. This gives rise to objectionable gear noises or unwanted vibrations of the drive chain.
In order to improve the maneuverability of the control lever, it has been proposed to incorporate into the lever a click mechanism which serves to clickingly hold the lever at each of predetermined pivotal positions, as disclosed for example in U.S. Pat. Nos. 4,744,265, 4,751,850, and 4,751,852 to Nagano. With the use of the click mechanism, any cyclist, skilled or unskilled, can easily operate the control lever to bring the shiftable cage or the guide plates accurately to an optimum position exactly corresponding to a selected sprocket of the multiple freewheel or the multiple chainwheel and thereby conduct desired speed change.
However, the proposed clicking type lever is not always advantageous and can sometimes become troublesome for example when the length of the control cable unexpectedly increases due to repeated use because such a cable elongation results in a positional deviation of the shiftable cage or the guide plates from an intended gear position corresponding to each clicked pivotal position of the lever. Although this positional deviation can be corrected by readjusting the cable length, such readjustment requires considerable time and skill.
The same problem also occurs when the multiple freewheel or the multiple chainwheel is replaced by another having the same number of sprockets at different spacing. In fact, multiple gears now commercially available include not only ones with constant spacing but also ones with varying sprocket spacing. The clicking mechanism, on the other hand, provides clicking positions only at fixed spacing, and it is impossible to hold the lever at a pivotal position between two adjacent clicking positions because of insufficient friction.
According to the three U.S. patents mentioned above, it is possible to switch between a frictional mode and a click mode by operating a changeover mechanism. In the frictional mode, the control lever can be held at any pivotal position by a rotational friction which is large enough to overcome the tension of the control cable. This frictional mode is advantageous in that the pivotal position of the control lever can be adjusted in any way to properly control the deraileur even at the time of cable elongation or after gear replacement. In the click mode, the lever, which is now free of strong rotational friction, is held only at predetermined pivotal positions by clicking engagement which is strong enough to overcome the tension of the control cable. The cyclist is thus allowed to enjoy the benefits of both operational modes depending on his or her own selection. However, it is impossible to enjoy the benefits of both operational modes at the same time.
U.S. Pat. Nos. 4,699,018 and 4,768,395 to Tagawa (the inventor of the present invention) also disclose a bicycle lever assembly which enables similar selection between a frictional mode and a clicking mode.